fix: Disabled Appearance="Primary" button renders white text on light background

[Copilot]

Since 4.2.1, a ui:Button with Appearance="Primary" IsEnabled="False" renders white text on the grey disabled background — invisible in light theme.

Pull request type

• Update
• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes

What is the current behavior?

WPF Style triggers have higher precedence than ControlTemplate triggers for the same dependency property. The Appearance="Primary" Style trigger sets Foreground = AccentButtonForeground (white). When the button is also disabled, the ControlTemplate's IsEnabled=False trigger attempts to set Foreground = ButtonForegroundDisabled, but is overridden by the Style-level Appearance trigger — leaving white text on a grey disabled background.

What is the new behavior?

• An IsEnabled=False trigger is added at the end of Style.Triggers in DefaultUiButtonStyle, after all Appearance triggers.
• Since WPF resolves conflicting Style triggers in declaration order (last wins), this ensures ButtonForegroundDisabled always overrides the Appearance foreground when the button is disabled — for all appearances.

<!--  DISABLED - must be last to override Appearance trigger foreground  -->
<Trigger Property="IsEnabled" Value="False">
    <Setter Property="Foreground" Value="{DynamicResource ButtonForegroundDisabled}" />
</Trigger>

Other information

Regression introduced by #1680, which moved the disabled foreground setter from per-element TargetName setters (inside ControlTemplate.Triggers) to a Style-level Foreground setter — exposing the trigger precedence conflict with Appearance triggers.

[dkuaf]

i think it should be better to move the Appearance triggers to inside the ControlTemplate.Triggers and position them before the existing triggers, i tested it and it works nicely. @pomianowski can i make another PR?